Methods of measuring the velocity of an aircraft consists in measuring the frequency shift between an emitted reference wave and a wave backscattered by the natural aerosols in the air, which are used as velocity tracers, located on the line-of-sight axis. This shift is representative of the projection, on the line-of-sight axis, of the velocity of the aircraft relative to the air. To take reference measurements, the laser is generally focussed onto a focal zone chosen to be sufficiently far from the aircraft so that the mass of air contained therein is as little disturbed by the aircraft as possible. In this case, the backscattered wave emanates mainly from the tracers within the focal zone.
The useful information carried by the Doppler frequency shift is obtained by using coherent detection. A wave emanating from a coherent light source is split into two. A first wave, called “emitted beam”, is sent into the focal zone and a second wave, called “local oscillator”, constitutes a reference for coherent detection of the Doppler frequency shift.
The aerosols naturally present in the atmosphere backscatter the light from the emitted beam. The emitted beam is backscattered with a frequency shifted by a Doppler shift Δfi relative to the frequency of the emitted beam. The backscattered signal is mixed with the reference wave to form a heterodyne beat signal on the photosensitive surface of a detector.
The Doppler shift Δfi, on a line-of-sight axis Xi, is the difference between the frequency fi of the backscattered signal and the frequency f of the local oscillator. It is used to deduce a measurement of the projection, on the line-of-sight axis Xi, of the velocity of the aircraft relative to the ambient medium, i.e. relative to the mass of air lying within the focal zone. The expression that links these two quantities is the following:vi=Δfi·λ/2  (A)
vi being the projection, on the line of sight Xi, of the velocity vector of the aircraft relative to the mass of air lying on the line-of-sight axis or in the focal zone and
λ being the wavelength of the beam emitted in the medium.
The Doppler shift Δfi on a line of sight Xi is conventionally obtained by analysing the backscattered signal. Among the methods that can be used, Fourier transform spectral analysis is generally employed.
The velocity vector of an aircraft is calculated from the components vi of the velocity vector along three non-coplanar line-of-sight axes, the components of the velocity vector vi being obtained using equation (A) from the Doppler shifts Δfi of the backscattered signals on the line-of-sight axes in question. This velocity vector is expressed as the TAS (True Air Speed) and the AOA (Angle Of Attack) and SSA (Side Slip Angle).
The frequency of the backscattered signal is determined from the frequency of the line present in the spectrum of the backscattered signal.
However, when an aircraft moves through a mass of air which is speed-inhomogeneous (that is to say when there are several masses of air moving at different speeds), it is not possible to determine the components of the velocity vector on the line-of-sight axes reliably since it is sometimes impossible to determine a unique frequency in the spectra of the backscattered signals on the line-of-sight axes. This is the case when there are distant clouds, when there is turbulence or ground echoes or when there are perturbations of the wake vortex.
The conventional methods of determining the velocity of an aircraft have the drawback either of determining a velocity which may be false (the pilot being unaware that the value supplied to him is not reliable) or of not determining a velocity vector (the velocity of the aircraft is therefore not accessible).
The aforementioned drawbacks are specific to continuous lasers emitting a narrow spectrum. A method based on a pulsed laser does not have these drawbacks, but this method is more expensive and less precise, for velocity measurements, than a method based on a continuous laser since the spectra resulting from the backscattered signals are spectrally more spread out.